This invention relates to bubble ink jet printing systems and, more particularly, to an integrated circuit chip which contains active driver, logic and resistive heater elements.
Thermal ink jet printers are well known in the prior art as exemplified by U.S. Pat. Nos. 4,463,359 and 4,601,777. In the systems disclosed in these patents, a thermal printhead comprises one or more ink filled channels communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A plurality of thermal energy generators, usually resistors, are located in the channels at a predetermined distance from the nozzle. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expells an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
In order to generate the resistor current pulses, some type of active drive device must be employed. Preferably, the drive circuitry should be formed on the same chip as the resistive elements. Prior art printheads use transistors having both positive and negative charge carriers (bipolar circuitry). Examples of printheads using bipolar drive circuits combined on the same chip as the resistors are provided in U.S. Pat. Nos. 4,251,824, 4,410,899 and 4,412,224. The disadvantages of the bipolar drive circuit prior art devices is that they require an expensive manufacturing technique and that they provide an inefficient use of the thermal energy. If the printhead could be made more thermally efficient, then less expensive MOS type circuitry (N-MOS) can be used to drive the head instead of the more expensive bipolar circuitry. Additionally, bipolar transistors exhibit destructive thermal run away when switching high currents. It is, of course, desirable and cost effective to have a resistor structure which is immediately and simply integrated on the same wafer with MOS drive electronics, preferably without additional process steps.
Prior art devices which utilize MOS type circuitry are disclosed in U.S. Pat. Nos. 4,595,821 to Matsuera and 4,532,530 to Hawkins.
In the Matsuera patent, a thermal printhead is disclosed in which a CMOS control circuit is mounted on a ceramic substrate upon which is also mounted the resistor element. This configuration is not suitable for thermal ink jet printing applications. In the Hawkins patent, a thermal printhead is formed, as the FIG. 4A and 4B embodiments. A polycrystalline silicon is simultaneously used to form the resistor and the associated drive circuitry. This configuration requires that each resistor element be attached to a bonding pad by a separate utilization contact process. Factors such as cost, more limited reliability and ink jet chip parameter space required for a bonding pad associated with every resistor, are disadvantages for moderate and high speed printing applications and for printing at a density near or above 300 spi.
According to a first aspect of the present invention, a monolithic silicon semiconductor integrated chip incorporating both MOS transistor drivers and resistive heater elements is provided which is more reliable in operation and more compact than prior art devices. According to a second aspect of the invention, semiconductor NMOS fabrication techniques are improved so as to increase transistor driver breakdown voltage and to add thermal efficiency while decreasing chip size.
The present invention is therefore directed to an improved monolithic silicon integrated circuit chip for ink jet printing which incorporates MOS drive circuitry and resistive heater elements. More particularly, the invention relates to a monolithic silicon integrated circuit chip for use with a bubble jet ink printhead,
said chip comprising a plurality of polysilicon gated MOS transistor switches electrically connected to a plurality of polysilicon resistive elements,
said resistive elements formed on a thermally grown field oxide layer having a thickness ranging from about 1 to 4 microns.